Scoliosis is a spinal deformity characterized by an abnormal curvature of the spine in the coronal plane and is often associated with an abnormal rotation in the axial plane causing ribs to protrude posteriorly into what is commonly referred to as “rib hump”. Adolescent idiopathic scoliosis (AIS) is the most prevalent type of scoliosis which develops during adolescence in an otherwise healthy patient and typically ceases at the onset of skeletal maturity. The cause of the disease is presently unknown.
Current surgical treatment of scoliosis involves manipulation of the spinal column and attachment of corrective devices for fusion of a portion of the spine. One such system, the Cotel-Dubousset system, disclosed in U.S. Pat. No. 5,147,360 to Dubousset, utilizes rigid metal rods attached to the spine. The rods are manipulated during surgery in an attempt to reduce abnormal curvatures and rotations of the spinal column. Typically, extensive discectomies are necessary, as well as removal of the spinous processes and injury to the spine itself to induce bleeding to aid bone fusion. The spine is then fused with bone graft harvested from the patient's illium or from a bone bank.
The surgery is arduous, invasive, and has an array of potential complications. Large loads are exerted on the spine for correction [1] which risks the patient's neurological condition. A long incision is required and extensive bone graft is harvested, therefore excessive blood loss can occur. Recovery can be a lengthy and painful process. If normal lordosis and kyphosis are not restored, a condition called “flat back syndrome”, the patient may have chronic pain. Even a successful procedure rarely results in a normal spinal curvature and the patient is left with an immobile spinal section. The discs above and below the fusion zone are in jeopardy of degeneration due to the increased biomechanical demands placed on them. In general, it is a major surgery with the possibility of major complications.
Therefore, it is evident there are flaws in prior art methods and devices. Most prior art devices are part of the load path of the spinal column. For example, it is understood that the Cotel-Dubousset system rigidly attaches stiff stainless steel rods to the spine. A structure having two roughly parallel support members relies primarily on the stiffer of the two members for transmission of loads. Therefore, loads exerted on an instrumented spine are transferred through the implant instead of through the spine. Spinal loads can be significantly large, and the geometry of the implants used is such that they will not support such loads indefinitely. Fatigue failure of the implant will occur if fusion is delayed.
The mechanical properties of spinal structures such as the intervertebral discs, ligaments, nerves and muscles have a time-dependent relationship between force and displacement, a characteristic called viscoelasticity. Viscoelastic structures increase strain under the action of an applied constant stress (creep) and decrease internal stress under the action of an applied constant strain (stress-relaxation). It has been shown that dramatic stress-relaxation occurs over time in patients who have undergone scoliosis correction surgery involving stiff metal rods. However, the stiffness of the rods prevents them from taking advantage of this phenomenon.
U.S. Pat. No. 5,490,851 to Nenov et al. describes a pelvis alignment device which rigidly attaches to the pelvis to align the pelvic girdle. After alignment with this device, artificial joints are used to hold the pelvis and sacrum in the aligned position. Flexible cords with worm gear drive members are attached so that “force is applied to the distorted spine so that it does not tend to act against the effects of the joints”. Therefore, the flexible cords work in conjunction with the artificial joints to maintain alignment correction achieved with a pelvis alignment device.
U.S. Pat. No. 6,849,076 to Blunn et al. describes a group of magnet driven devices for surgical distraction. A device for correction of scoliosis is described which is attached to two vertebrae and is non-invasively distracted by an external changing magnetic field.
U.S. Pat. No. 6,299,613 to Ogilvie et al. describes a tether connected to points on the spine to maintain a curvature, or to constrain growth of the convex side of the spine.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.